The Vav family is a new group of oncoproteins with known representative in vertebrates (Vav, Vav-2, and Vav-3) and nematodes (C. Elegans Vav). These proteins work as enzymes that catalyze the exchange of nucleotides on GTP-binding proteins of the Rho/Rac family, thereby facilitating the transition of these GTPases from their inactive (GDP-bound) to their active (GTP-bound) to their active (GTP- bound) state. The enzyme activity of Vav proteins is tightly regulated during signal transduction. In the absence of stimuli, these proteins remain inactive due to low phosphorylation levels. After cell stimulation, Vav proteins become activated by direct phosphorylation on tyrosine residues, leading to the stimulation of Rho/Rac-dependent intracellular pathways. Several lines of evidence demonstrate that the function of Vav proteins is crucial for mounting effective developmental and mitogenic responses. Thus, the deletion of the vav gene by homologous recombination results in impaired lymphoid development, lymphopenia, and in defective immune responses of mature B-and T-lymphocytes. Furthermore, the ectopic express of gain-of-function mutants of Vav and Vav-2 induces high levels of cellular transformation in rodent fibroblasts. Finally, recent results have implicated the Vav pathway in the pathogenic cycle of human lymphotropic viruses such as HIV and HTLV. The long-term objective of our laboratory is to achieve a comprehensive characterization of this protein family at the biochemical, cellular, and organism level. To materialize this objective, we propose the following studies. In Aim 1, we will use biochemical and crystallographic techniques to visualize the conformational changes that trigger the activation of Vav proteins during physiological and tumorigenic conditions. In Aim 2, we will perform site-directed mutagenesis studies to characterize the structural determinants that mediate the interaction of Vav proteins with Rho/Rac GTPPases. In Aim 3, we will utilize expression cloning approaches to isolate inhibitory and synergistic factors that modulate the activity if Vav proteins in vivo. In Aim 4, we will use homologous recombination technology to investigate the role of each Vav family member, and of the Vav family as a whole, in developmental and physiological responses. These studies should give us for the first time should give us for the first time a unified vision of the modus operandi of Vav proteins during signal transduction. This will in turn contribute to a better understanding of the intracellular pathways that mediate the proliferation of normal and cancer cells.